This invention is concerned with the utilization of two industrial by-products, namely Class F fly ash and cement kiln dust (hereinafter CKD). Fly ash is the very finely divided ash produced by the combustion of antracite and bituminous coal in large industrial coal-fired boilers, especially for the steam generation of electricity, that is suspended in the flue gases from such boilers and is separated therefrom by e.g. electrostatic precipitation. CKD, on the other hand, is a very finely divided dust produced during the manufacture of portland cement in high temperature usually rotary furnaces which is carried off in the flue gases from such furnaces and similarly separated therefrom. Both of these by-products are being generated in enormous quantities in the order of millions of tons per year and are an increasing serious environmental problem. There thus exists an important need for economically effective ways of consuming these otherwise waste materials.
Details as to the nature of Class F fly ash as well as CKD, typical sources of the same and other relevant information are set forth in my co-pending application Ser. No. 07/229,454, filed Aug. 8, 1988, now U.S. Pat. No. 4,992,102 issued on Feb. 12, 1991, the contents of which are incorporated in entirely by reference into this disclosure. That prior application was based on the discovery that if Class F fly ash and CKD are blended together in proper proportions, generally in the range of about 40:60 to 60:40 by wt, the resultant blend meets the requirements set up by ASTM specification C-618-85 for a Class C fly ash. Natural Class C fly ash is derived from the combustion of lignite and subbituminous coals which are available in certain limited regions of the United States and is known to possess more desirable properties than Class F fly ash is are consequently recognized as a substantially more valuable commodity. In effect, the combustion of CKD in judicious amounts with Class F fly ash according to the prior disclosure upgraded the Class F fly ash to the Class C fly ash category, as characterized by ASTM Standard C-618-85, creating a "synthetic" Class C fly ash blend. The details of ASTM C-618-85 appear in my prior application identified above.
That prior application was also concerned with the use of the new "synthetic" Class C fly ash blend as an effective replacement for portland cement in all purpose or general purpose concrete mixes, whether prepared in bulk, as ready mixes or in bags. Thus, replacement of at least about 25% up to about 50% by wt of portland cement in otherwise conventional general purpose concrete mixes, up to a maximum of about 200 lbs in certain high strength mixes, yielded a general purpose concrete exhibiting a compression strength (the most important property of general purpose concrete) substantially comparable to corresponding all-portland cement mixes and substantially greater than exhibited by similar mixes containing lesser amounts, e.g. 20 by wt., of Class F fly ash alone, i.e. absent any CKD. Since both Class F fly ash and CKD were available at a fraction of the cost of portland cement, this result had an obvious economic appeal, apart from the potential environmental benefit of disposing of considerable quantities of major waste products.
The emphasis of the prior application was on utilizing as much of the Class F fly ash and CKD as possible, thereby maximizing both the favorable cost and environmental consequences, which led to the selection of a minimum replacement level for portland cement of about 25% by wt. Further experience with the inclusion of these materials in general purpose concrete mixes has established that a lesser replacement, i.e. below about 25% by wt, of the portland cement in such mixes is also advantageous. One would naturally expect that with increasing amounts of portland cement, the levels of compression strength in the general purpose concrete produced from such mixes would remain high. What was unexpected in the results achieved with mixes containing a lesser amount than 25% by wt of Class F fly ash and CKD together has been an impressively high level of early compression strength, that is the compression strength exhibited by the general purpose concrete during the period immediately following pouring of the same, especially day 1 through day 3, and even up to day 7, which even exceeded the values observed in corresponding all portland cement mixes.
Virtually always, general purpose concrete is poured within forms or frames for support thereby until the concrete hardens to a sufficient degree to permit the forms to be removed. For most construction work with general purpose concrete, economic considerations demand that the forms be removed as soon as possible so that they can be moved to the next location at the site for the next pouring. For building construction, this may be the next floor to be erected on top of the previous floor so that the latter must possess sufficient strength to support new floor above it. The forms cannot, therefor, be moved until adequate development of strength is assured. The collapse during construction of buildings constructed of concrete due to premature removal of the construction forms before adequate strength development is an occasional occurrence, sometimes of a disastrous nature, in the building field. High early strength portland cements have been developed to address this problem, being identified as ASTM Type III cement, but they tend to be more expensive.
As set forth in the prior application, the incorporation of Class F fly ash in concrete was up to about a 20% replacement level of the portland cement is often permissible under local construction regulations. But it is widely recognized that this practice significantly impairs the early strength of the resultant concrete. As shown by Lea, "The Chemistry of Cement and Concrete", copyright 1971, Chemical Publishing Company, New York, N.Y., at Page 437 and 438, when 20% by wt portland is replaced by "pulverized fuel ash", the author's synonym for fly ash, the 7-day strength is reduced by 10-30%, while the 1-day and 3-day values are at least equally impaired if not somewhat worse. While the strength of the concrete is usually recovered over time, i.e. from about 90 days to one year, this is of little or no consolation where high early strength is critical. It appears to be quite unexpected that the combination of CKD with Class F fly ash for replacing less than 25% portland cement can at least substantially compensate for normal propensity of Class F fly ash to materially reduce the early strength in concrete containing the same in comparable amounts and in certain proportions to actually over compensate for the strength loss and impart higher early strengths than for corresponding all portland cement mixes. While the use of smaller amounts of these waste products cannot offer the promise of consuming as much of the available supplies as did the prior concept, it does extend that consumption into otherwise unavailable areas of use and may therefore contribute additionally to their beneficial utilization.